A  CONTRIBUTION  TO  THE  OOLITE  PROBLEM 


I 


/ 

FRANCIS  M.  VAN  TUYL 


Reprinted  for  private  circulation  from 
The  Journal  of  Geology,  Vol.  XXIV,  No.  8.  November-December  1916 


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A  CONTRIBUTION  TO  THE  OOLITE  PROBLEM 


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FRANCIS  m:  van  tuyl 

University  oi  Illinois 


INTRODUCTION 


At  the  present  time  there  are  two  prevalent  theories  of  oolite 
formation,  namely,  the  inorganic,  or  chemical  precipitation  theory, 
and  the  organic  theory.  Prior  to  the  year  1890  the  inorganic  theory 
was  generally  agreed  to  and  it  is  to  this  day  the  most  widely  accepted 
of  the  two. 

In  the  year  mentioned,  however,  Wethered1  pointed  out  a  close 
relationship  between  the  concretionary  structure  of  the  calcareous 
algae  Girvanella  and  that  of  true  oolite,  and  showed  that  certain 
so-called  oolites  of  the  Carboniferous  and  Jurassic  of  England  really 
consist,  in  part  at  least,  of  rounded  calcareous  masses  secreted  by 
this  organism,  since  they  possess  in  addition  to  the  concretionary 
structure  the  vermiform  tubules  which  characterize  the  genus.  But 
in  this  and  again  in  a  succeeding  paper,  entitled  “The  Formation 
of  Oolite,”  which  appeared  in  1895/  Wethered  was  unable  to  demon¬ 
strate  the  presence  of  the  Girvanella  tubules  in  typical  oolite  spher¬ 
ules  showing  both  radial  and  concentric  structure,  although  he  was 
led  to  believe  that  these  were  also  of  algal  origin. 

Following  closely  upon  Wethered  as  a  champion  of  the  organic 
theory  came  Rothpletz,  who  published  a  paper  on  the  origin  of 
oolite  in  189 2. 3  This  investigator  upon  studying  the  recent  oolites 
of  Great  Salt  Lake  found  that  where  these  were  still  in  the  water 
they  were  usually  covered  by  a  bluish-green  algal  mass  consisting 
of  the  cells  of  Gloeocapsa  and  Gloeothece ,  forms  which  are  known  to 
secrete  carbonate  of  lime;  and,  when  the  oolite  grains  and  rodlike 


1  Quar.  Jour.  Geol.  Soc.  London,  XL VI,  270-83. 

2  Ibid.,  LI,  196-209. 

*Botanisches  Centralblatt,  No.  35,  pp.  265-68  (English  translation  by  F.  W.  Cragin, 
American  Geologist,  X,  279-82). 


792 


A  CONTRIBUTION  TO  TEE  OOLITE  PROBLEM 


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calcareous  bodies  on  the  shore  were  dissolved  in  acid,  they  all  yielded 
dead  and  shriveled  fission  algae.  Rothpletz,  therefore,  concluded 
that  the  oolites  of  Great  Salt  Lake  are  the  product  of  lime-secreting 
fission  algae,  and  that  their  formation  is  proceeding  day  by  day. 

Furthermore,  a  study  of  the.  recent  and  near-recent  oolites  of 
the  Red  Sea  showed  these  also  to  contain  minute  grains  of  organic 
material  suggesting  fission  algae.  But  these  differ  from  the  Great 
Salt  Lake  oolites  in  that  their  nuclei  always  consist  of  sand  grains 
and  in  that  their  concentric  structure  is  less  well  developed.  They 
also  possess  small  vermiform  canals  filled  with  calcite,  which  are 
interpreted  as  imprisoned  algae  of  another  type. 

Rothpletz  also  remarks  that  certain  elongated  corpuscles  pos¬ 
sessing  oolitic  structures,  which  he  interprets  as  organic,  occurs  in  the 
Lias  limestone  of  the  Vilser  Alps,  and  concludes  as  follows :  “Accord¬ 
ing  to  the  present  stage  of  my  researches,  I  am  inclined  to  believe 
that  at  least  the  majority  of  the  marine  calcareous  oolites  with 
regular  zonal  and  radial  structure  are  of  plant  origin ;  the  product  of 
microscopically  small  algae  of  very  low  rank,  capable  of  secreting 
lime.” 

In  spite  of  these  discoveries  by  Wethered  and  Rothpletz,  later 
students  of  the  oolite  problem  have  tended  to  drift  back  to  the 
inorganic  theory  and  to  regard  the  association  of  oolites  with  algae 
as  accidental.  Thus  Linck1  has  shown  by  experiment  that  oolites 
similar  to  natural  ones  may  be  produced  artificially  by  the  action 
of  sodium  carbonate  and  ammonium  carbonate  on  the  calcium  sul¬ 
phate  of  sea-water.  He  points  out  that  these  carbonates  are  formed 
by  decomposition  of  animal  and  plant  tissues  in  the  sea,  and  favors 
the  view  that  oolites  have  been  formed  in  this  way.  That  natural 
oolites  can  be  formed  chemically  is  demonstrated  by  Vaughan,2 
who  points  out  that  oolitic  structure  is  now  being  developed  in  the 
calcareous  muds  precipitated  through  the  agency  of  bacteria  off  the 
coasts  of  Florida  and  the  Bahamas. 

In  a  recent  review  of  the  whole  question  of  oolite  formation, 
T.  C.  Brown3  has  endeavored  to  substantiate  Linck’s  conclusions 

1  Neues  Jahrb.,  Beil.  Bd.  16  (1903),  pp.  495-513. 

2  Jour.  Washington  Acad.  Sci.,  Ill  (1913),  302-4. 

3  Bull.  Geol.  Soc.  America,  XXV  (1914),  745-8o. 


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FRANCIS  M.  VAN  TUYL 


and  to  discount  the  importance  of  the  algal  theory.  To  quote  from 
him:  “The  dead  algal  cells  in  the  Salt  Lake  oolite  are  regarded  as 
cells  which  had  selected  the  oolite  as  a  point  of  attachment.  They 
became  imprisoned  within  it  by  the  further  accretion  of  aragonite 
by  chemical  precipitation.”  He  suggests  that  the  decay  of  the 
attached  algae  furnishes  Na2C03  which  acts  as  a  precipitating  agent 
and  thereby  aids  the  growth  of  the  oolite. 

As  regards  the  importance  of  algae  in  the  production  of  the 
oolites  of  Great  Salt  Lake,  future  studies  may  be  expected  to  throw 
additional  light  on  the  problem.  Microscopic  examination  of 
these  by  several  investigators  has  failed  to  reveal  any  indications 
of  algal  structure  in  the  calcareous  grains  themselves.  On  the  other 
hand,  they  exhibit  highly  developed  radial  and  concentric  structure. 

THE  PRAIRIE  DU  CHIEN  OOLITE 

Some  time  ago  the  writer  had  occasion  to  examine  microscopi¬ 
cally  a  siliceous  oolite  which  marks  the  base  of  the  Ordovician  in 
northeastern  Iowa,  and  found  to  his  surprise  that  the  oolite  grains 
of  this  showed  undoubted  algal  structures.  The  bed  in  question 
constitutes  the  so-called  transition  member  between  the  Prairie 
du  Chien  dolomite  and  the  Saint  Croix  sandstone.  With  reference 
to  this  bed  Leonard,  in  his  “Geology  of  Clayton  County,”  says: 

The  lower  Magnesian  is  not  marked  off  sharply  from  the  underlying  Saint 
Croix,  but  there  is  a  transition  from  the  one  to  the  other  through  from  fifteen 
to  twenty  feet  of  calcareous  sandstone  or  siliceous  oolite.  The  rock  is  com¬ 
posed  of  clear  rounded  grains  of  quartz  cemented  by  lime  carbonate.  In 
some  beds  this  cementing  material  is  quite  abundant,  in  others  there  is  only 
enough  to  hold  together  the  grains.  The  ledges  vary  in  thickness  from  a 
few  inches  to  two  or  three  feet.  This  siliceous  oolite  is  well  exposed  in  an  old 
quarry  in  the  river  bluff  one  and  one  half  miles  above  North  McGregor.  The 
transition  beds  are  also  seen  in  the  section  at  Point  Ann,  just  below  McGregor. 
Here  there  are  alternating  layers  of  sandstone  and  limestone  and  some  oolite 
similar  to  that  described  above.1 

A  bed  of  similar  character  and  thickness  has  been  described  by 
Calvin2  as  occurring  at  the  same  horizon  in  Allamakee  County, 
which  lies  directly  north  of  Clayton.  The  writer  has  examined 

1  Iowa  Geol.  Survey,  XVI  (1905),  239-40. 

2  Ibid.,  IV  (1894),  61. 


A  CONTRIBUTION  TO  TEE  OOLITE  PROBLEM 


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the  member  at  the  Point  Ann  exposure  only,  and  the  samples  here 
described  and  figured  are  entirely  from  that  locality. 

Microscopic  examination  of  the  rock  shows  it  to  consist  of 
imperfectly  preserved  siliceous  oolite  grains  in  a  dolomitic  matrix. 
The  history  of  the  rock  is  briefly  as  follows:  Subsequent  to  the 
formation  of  the  oolite,  dolomitization  set  in,  transforming  the  cal¬ 
careous  matrix  completely,  and  many  of  the  calcareous  oolite  grains 
either  wholly  or  in  part,  to  dolomite.  Alteration  then  ceased  and 
silicification  of  the  unchanged,  or  only  partly  changed,  oolite  grains 
ensued.  The  irregular  areas  of  dolomite  within  the  interiors  and 
the  frayed-out  borders  of  many  of  the  silicified  oolite  grains  are  i 
this  way  accounted  for.  The  structure  of  grains  which  were  com¬ 
pletely  dolomitized  prior  to  silicification  is  almost  entirely  obliter¬ 
ated,  and  these  are  often  only  with  difficulty  distinguished  from  the 
matrix. 

The  oolite  grains  range  from  o.  i  mm.  to  i .  13  mm.  in  diameter, 
and  when  well  preserved  show,  in  addition  to  the  concentric  and 
radial  structure,  minute  sinuous,  enwrapping  fibers  very  similar  to 
the  tubules  which  characterize  the  Girvanella  type  of  calcareous 
algae.  A  comparison  of  the  microphotographs  of  the  oolite  grains 
with  that  of  Girvanella  problematica  Nicholson,  described  and  figured 
by  Rothpletz,  in  his  memoir  entitled  “Ueber  Algen  und  Hydrozoen 
im  Silur  von  Gotland  und  Oesel,”1  will  bring  out  this  striking 
similarity  (Figs.  1-6). 

It  should  be  recognized  that  the  interwoven  fibers  of  the  oolite 
have  been  partly  obliterated  by  silicification.  Doubtless  these  con¬ 
sisted  of  hollow  tubules  filled  with  calcite,  like  those  shown  by  Girva¬ 
nella  problematica  prior  to  silicification. 

The  fibers  of  the  organism  of  the  oolite  have  an  average  diameter 
of  0.015  mm.  which  agrees  very  closely  with  the  diameter  of  the 
tubules  of  Girvanella  problematica ,  which  varies  from  0.01  to 
0.018  mm.,  according  to  Rothpletz. 

Typically  the  well-preserved  oolite  grains  consist  of  an  inner 
structureless  nucleus,  followed  by  a  narrow  intermediate  band 
showing  radial  structure,  and  this  again  by  an  outer  band  bearing 

1  Kungl.  Svenska  Velenskapsakademiens  Handlingar ,  Band  43,  No.  5  (1908), 
PI.  I,  Fig.  1. 


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Fig.  i. — Microphotograph  of  Girvanella  problematica  Nicholson.  About  X42. 
After  Rothpletz. 

Fig.  2. — Microphotograph  of  peripheral  section  of  a  silicified  oolite  grain  from 
basal  Ordovician  at  McGregor,  Iowa.  About  X45. 

Fig.  3. — Cross-section  of  another  grain  from  the  same  locality.  About  X45. 
Note  the  well-developed  algal  structure  in  the  outer  portion  and  the  band  showing 
radial  structure  within  this.  The  interior  is  not  preserved. 

Fig.  4. — Imperfectly  preserved  oolite  grain.  About  X45.  The  interior  and 
peripheral  portions  of  the  grain  were  replaced  with  dolomite,  with  obliteration  of  struc¬ 
ture,  prior  to  silicification. 

Fig.  5. — Silicified  grain  showing  well-developed  radial  structure  but  with  algal 
fibers  nearly  obliterated.  About  X45. 

Fig.  6. — Another  grain  showing  fine  concentric  structure  but  with  no  distinct  algal 
fibers  preserved.  About  X45. 


A  CONTRIBUTION  TO  THE  OOLITE  PROBLEM 


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sinuous  fibers.  In  some  instances,  however,  the  two  outer  bands 
grade  gradually  into  each  other  without  any  distinct  line  of  demarka- 
tion;  or  indeed  the  radial  structure  may  be  entirely  wanting  and  the 
concentric  structure  may  continue  into  the  nucleus.  The  fibers  are 
best  shown  in  peripheral  sections  of  the  grains.  In  these  they 
appear  to  enwrap  the  bodies. 

Some  of  the  grains,  however,  show  little  or  no  trace  of  algal 
fibers,  but  there  is  convincing  evidence  that  this  fact  has  resulted  in 
most,  if  not  all  cases,  from  the  obliteration  of  original  structures  as 
an  accompaniment  of  silicification.  All  stages  of  such  obliteration 
may  be  traced  under  the  microscope. 


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